Modern silicon CCD and CMOS image sensors are becoming extremely small, so that the package size of commercial miniature cameras is becoming dominated by the image forming optics, even if the latter is only a structure supporting a pinhole. Recently, there have been demonstrations of ultra-flat, extremely light weight cameras working in the visible region of the spectrum. Such cameras are inspired by biological imaging systems. Similar ideas for cameras developed to operate in the infra-red would help to drastically reduce the size, weight and cooling requirements in many imaging applications, also offering substantial cost reductions. In addition, wide field-of-view designs can potentially simplify control systems that rely upon near infra-red imaging.
It is desirable to produce a near infra-red (NIR) imaging device with the following characteristics
a capability to operate at 1064 nm.
a total field of view of 120°
a pixel resolution of ≦3 mrad
A biologically inspired imaging device which is inspired by the construction of eyes in insects consisting of a segmented, convex sensor coupled to a similarly shaped microlens array might meet these requirements. To minimize size, this design would require a number of small individual sensor arrays to be closely packed at specific angles. However, such sensor design and manufacture is likely to be costly. In the longer term, sensors fabricated on flexible polymer substrates might provide a viable solution, but it is currently desirable to provide an imaging device which can use a conventional single planar sensor.
It is possible to use a planar sensor if beam deflection optics are used. If the beams are deflected prior to entering the lenses then a planar arrangement of lenses may also be used. One example in the holographic lenses proposed by Tai et al in ‘Design and Fabrication of a Wide-Field Holographic Lens for a Laser Communication Receiver’, A. M. Tai, M. T. Eismann and B. D. Neagle. Opt. Eng. 32 (12) 1993 pp. 3254 3266. This is illustrated schematically in FIG. 1 where a planar sensor 1 receives beams A, B, C via lenses 2 after deflection of beams A, C, by holograms 3. However in order to obtain high deflection efficiencies over a wide field of view it would be necessary to further segment the holograms; this again would be complex and costly.
As an alternative to holograms, prisms may be used either to deflect the beams prior to entering the lens as illustrated schematically in FIG. 2 where a planar sensor 1 receives beams A, B, C via lenses 2 after deflection of beams A, C, by prisms 4. Alternatively prisms may be provided at the sensor surface. However, prisms are bulky for manufacture and will cause chromatic aberrations. Furthermore, deployment at the sensor surface will tend to generate additional coma and astigmatism due to the converging beam as described in ‘Formulas for the Coma and Astigmatism of Wedge Prisms used in Converging Light’, J. W. Howard. App. Opt. 24 (23) 1985, pp. 4265-4268.
Therefore it is desired to produce an inexpensive miniature imaging device which has the characteristics described above.